if the question said 2-x is greater OR EQUAL TO 6, your answer (-4) is perfectly right.
however, since the expression says 2-x is greater (but not equal to) 6, the largest possible value would be 3.99999999999999999999999999999.......... in other words, it is infinitely close to 4 but not quite exactly 4. So there is no single largest value for x.